1. Field of the Invention
The present invention relates to a golf ball comprising a spherical outer-layer core having a spherical inner cavity and a spherical inner-layer core disposed within the spherical cavity, wherein the outer core and the inner core rotate independently when the golf ball is traveling.
2. Related Art
Solid golf balls, such as two-piece golf balls, three-piece golf balls, etc. are usually produced by a process which comprises compression or injection molding for enclosing a solid core with a cover material and for forming dimples on the cover material, and finishing processing such as coating or mark stamping. In this case, a single-layer solid core or a multi-layer solid core composed of a plurality of solid layers has conventionally been used as a core of the solid golf ball.
In relation to the multi-layer solid core, the present inventors conceived a structure of a golf ball in which a spherical inner-layer core is disposed within a spherical cavity of a spherical outer-layer core, and a liquid is present in the space between the outer-layer core and the inner-layer core. With this structure, when the golf ball is traveling, the outer-layer core and the inner-layer core rotate independently and the inner-layer core rotates within the outer-layer core without receiving any air resistance, with the result that the golf ball can obtain a gyro moment. By virtue of this moment, the rate of spin is stabilized, the angle of fall becomes flat relative to the ground, travel distance is extended, and the straightness of the ball's trajectory is improved. Subsequently, the present inventors conducted a study in order to obtain such a golf ball.
As a result, the present inventors found that, depending on the thickness of the liquid layer between the outer-layer core and the inner-layer core, the inner-layer may not be able to rotate independently of the outer-layer core or lowers resilience of the golf ball; that, depending on the respective specific gravities of the inner-layer core and the liquid layer, the inner-layer core may not be able to rotate independently of the outer-layer core; and that, depending on the wall thickness of the outer-layer core, sufficient resilience and strength of the golf ball may not be secured.